Combining Different Animations/Skeletons
A mixamo animation, in order to work, has to share the same exact names between its channels and the bones of your rig.
In order to work well as expected the rotation rest pose of your rig should be the same as the rotation rest pose of the original Mixamo rig.
If this is not the case you have to perform a &quot;retargeting&quot;, which is too complex to explain in a short answer (there are tutorials, addons, end even manual procedures to perform).
When you reach theese conditions you can stack the mixamo animation on top of your previous animation with the NLA editor, turning off or overwriting the arms channels on your animation and turning off (or deleting) all non arms related channels in Mixamo animation.